Shaping of syntactic foam

ABSTRACT

A method of forming a shaped article from a stock of preformed, thermoplastic, syntactic foam material of hollow microspheres in a thermoplastic polymer matrix, comprising heating the preformed syntactic foam to a temperature above the softening point of thermoplastic polymer matrix and applying differential pressure over at least one surface of the foam material through the medium of a plastically deformable diaphragm member to cause the foamed material to conform to a desired shape. Shaped articles of syntactic foam material, optionally having a surface coating of unfoamed, preferably fibre reinforced thermoplastics materials are provided in a more versatile and convenient manner than curing or sintering syntactic foam material in a mould.

SHAPING OF SYNTACTIC FOAM

This invention relates to a method of forming shaped articles from asyntactic foam material.

There is a demand for high performance, light weight structuresparticularly in the aerospace industries. For example, improvements inradome construction are constantly sought. A radome is a cover for radartransmitting and receiving antennas, which protects the equipment fromthe environment during use, but permits microwave transmission with theminimum loss and distortion. In this, and other applications, asyntactic foam material, comprising hollow microspheres embedded in aresin matrix, is a useful component of construction but is limited interms of methods of fabrication because the microspheres are easilycrushed if subjected to compression. As a result shaped articles aregenerally made by in situ construction in a mould of the desired shapethus occupying a mould for a considerable time whilst materials arecured or sintered in the mould. It would be advantageous to be able toform a shaping from a preformed stock of syntactic foam material.

According to the present invention there is provided a method of forminga shaped article from a stock of preformed, thermoplastic, syntacticfoam material of hollow microspheres in a thermoplastic polymer matrix,comprising heating the preformed syntactic foam to a temperature abovethe softening point of thermoplastic polymer matrix and applyingdifferential pressure over at least one surface of the foam materialthrough the medium of a plastically deformable diaphragm member to causethe foamed material to conform to a desired shape.

The plastically deformable diaphragm which is used to urge the foamstock into the desired shape may be an organic thermoplastic materialwhich has a minimum forming temperature equal to or greater than theflow temperature of the thermoplastic matrix of the foam stock.Preferably, the diaphragm is of a plastically deformable metal anddesirably is a superplastically deformable metal as described inEuropean Patent Publication No. 155 820. When the diaphragm material isan organic, thermoplastic polymer it may be chosen so that it becomesintegral with the foam stock during the shaping of the article and formspart of the final shaped article. Suitable procedures are described inEuropean Patent Publication Nos. 195 561 and 195 562. When a metaldiaphragm is used it may be left in place after the shaping if it can besufficiently adhered to the foam stock to perform a useful function butin normal circumstances would be detached from the article after theshaping operation.

Preferably, the foam stock is shaped whilst confined between a pair ofdiaphragms.

Whether or not the shaping diaphragms are left in place after theshaping operation it may be useful to provide the foam stock with acoating of unfoamed and preferably, reinforced material which willconfer beneficial properties on the article such as strength, stiffness,resistance to rain erosion etc. This coating may be made integral withthe flat foam stock before it is shaped or may be laid up in the shapingmould in contact with the foam stock so that it becomes integral in theshaped article as a result of the shaping operation. In one embodimentthe coating consists of a fibre reinforced thermoplastic layer in whichthe fibres are continuous, collimated fibres, interwoven or overlaid toprovide quasi-isotropic reinforcement in the layer. Suitable fibrereinforced materials are described, for example, in European PatentPublication Nos. 56703 and 102 159. The process is operable usingrelatively low differential pressures because the foam stock is apreformed and already consolidated stock of hollow microspheres inthermoplastic.

The syntactic foam stock polymer matrix can be produced by known methodsbut it is preferred that the use of solvents to dissolve the polymer toaid dispersion of the microspheres should be avoided because of thedifficulty of removing residual solvent. It is preferred that themicrospheres be dispersed in molten polymer.

Suitable hollow microspheres may be obtained from Croxton and Garry Ltdof Dorking who supply a range of glass microspheres of various densitiesand crush strength. The grade B38/4000 is particularly suitable as ithas a high crush strength (nominally 4000 psi). Because of therelatively low pressures required in this process less densemicrospheres of lower crush strength may be used.

The method of the invention permits a suitable constructional materialhaving a syntactic foam core, optionally surfaced with a thermoplasticmaterial to be prepared under ideal conditions as a flat slab stock andto be subsequently thermoformed into a shaped article. Surprisingly themethod of the invention enables shaping to take place withoutsubstantial microsphere breakage.

The invention is further illustrated with reference to the followingexamples.

EXAMPLE 1

Polyetheretherketone (PEEK) having a melt viscosity of 100 Ns/m²measured at 400° C. at a shear rate of 1,000 sec⁻¹ and B38/4000(obtained from Croxton and Garry Ltd of Dorking) were blended dry at aratio of 2:1 by weight and then consolidated in a matched metal mould ofdimensions 6 in.×8 in. in a press capable of platen temperatures of 400°C. The top and bottom faces were separated from the metal mould withaluminium foil treated with Frekote mould release agent.

The mould and powder were heated to 400° C. for 30 minutes withoutpressure being applied. A pressure of 6 tonnes was then applied for fiveminutes. Platens and mould were then air cooled to 250° C. and then werecooled to room temperature at an average cooling rate of 15°/minute.

EXAMPLE 2

The procedure of Example 1 was repeated except in that the dry blend wasconsolidated in the mould between two skins each consisting of fourplies of unidirectionally reinforced prepreg of glass fibres inpolyetheretherketone (melt viscosity 100 Ns/m²). The four plies of eachskin were laid up to give a quasi isotropic distribution of fibres withthe unidirectional fibres of each ply being at 45° to the adjacent layer(ie +45°, 90°, -45°, 0°).

This process was repeated using unidirectional carbon fibre inpolyetheretherketone (melt viscosity 100 Ns/m². The flexural modulus andstrength of these sandwich structures were measured and are recordedbelow.

    ______________________________________                                                             Flexural   Flexural                                                           Modulus    Strength                                      Composite            GN/m.sup.2 MN/m.sup.2                                    ______________________________________                                        Glass fibre in PEEK skin/foam core                                                                 10.7       161                                           Carbon fibre in PEEK skin/foam core                                                                19         274                                           ______________________________________                                    

EXAMPLE 3

Shaping of the composites containing syntactic foam prepared in Examples1 and 2 is now described with reference to FIG. 1 which shows a typicalmoulding apparatus for use in the invention.

In FIG. 1 mating shells, 1 and 2 provide a chamber, shown generally as3, within which shaping operations can be performed. Shells 1 and 2 areprovided with heating means (not shown) so that chamber 3 caneffectively act as an oven of controlled temperature. Means forproviding pressure and vacuum are also provided (not shown). In FIG. 1the shells seal on metal diaphragms 4 and 5 composed of asuperplastically formable alloy. Diaphragms 4 and 5 are spaced apart byannular ring 6 corresponding in perimeter to the perimeter of shells 1and 2. Ring 6 is provided with a bore 7 disposed radially through theannulus to which a vacuum source (not shown ) can be connected so thatthe space between diaphragms 4 and 5 can be evacuated. Foamed stock 8 tobe shaped is shown located between diaphragms 4 and 5. In operationshells 4 and 5 are engaged with sufficient pressure to maintain a sealwhilst the diaphragm shaping proceeds. Vacuum is applied at 7 to removesubstantially all free air present between the diaphragms. Adifferential pressure is applied across opposing diaphragm faces, whilstthe diaphragms and the workpiece are at an elevated temperature. Afterthe desired shaping has been induced the shaping may be allowed to coolwithin the mould or the assembly of workpiece between the diaphragms maybe removed whilst still under vacuum as an umbilical pack for coolingoutside the mould.

Using an apparatus of the type described, having a shell outer diameterof about 190 mm, the product of Example 1 was formed between Supralalloy diaphragms (supplied by Superform Ltd of Worcester) using anannular separating ring 6 mm thick, with outer diameter 190 mm and innerdiameter 156 mm. The workpiece used was a disc of diameter 150 mm havinga thickness of 6 mm. The machine cavity 3 was preheated to 391° C.Shells 1 and 2 were then separated, and items 4, 5, 6 and 8 assembledcold. A vacuum of 720 mm Hg was drawn in the enclosed space, shells 1and 2 were engaged to seal the assembled items. The cold pack (items 4,5, 6 and 8) was allowed to gain heat for 16 minutes before applying airpressure to diaphragm 4, and the pack was removed after a total elapsedtime of 50 minutes. During this elapsed time the conditions varied asfollows:

    ______________________________________                                        Elapsed time                                                                             Temperature  Vacuum   Pressure                                     (mins)     (°C.) (mm Hg)  (bar)                                        ______________________________________                                        Zero       [391]        720      0                                             7                      600      0                                            16         394          600      0.35                                         19         394          600      0.7                                          24         389          600      1.05                                         32         Items 6, 7 opened and closed for in-situ                                      viewing of pack.                                                   36         389          575      1.4                                          40         Opened; pack removed for viewing; closed.                          42         389          575      1.75                                         44         389          575      2.1                                          46         389          575      2.45                                         48         389          575      2.75                                         50         Opened; pack removed for cooling.                                  57         Pack disassembled and item 8 released.                             ______________________________________                                    

The article 8 had a depth of draw of 32 mm. The surface quality was goodand little evidence of microsphere crushing was evident.

The procedure was repeated using the sandwich product of Example 2 inwhich the outer skins were glass fibre reinforced PEEK. The resultingshaping had a similar depth of draw. A further shaping was produced fromthe carbon fibre reinforced PEEK surfaced laminate of Example 2. Asimilar shaping of good surface quality was produced.

EXAMPLE 4

The procedure of Example 3 was modified by using an apparatus in whichthe formng chamber was provided by a pair of heated platens eachprovided with an annular band of matched dimensions. The bands attachedto the platens formed a chamber on being brought together at theirmatching perimeters. In addition, the lower half of the chamber wasprovided with a female mould having the shape of a circular ashtray ofinternal dimension approximately 28 mm deep and diameter 100 mm. Usingthis apparatus in the procedure of Example 3 a circular flat slab stockof material described in Example 1, of diameter 150 mm and having athickness of 2 mm was made into a pack between a pair of diaphragms ofpolyimide film 0.125 mm in thickness. The polyimide film used was`Upilex` R, supplied by Ube Industries Limited.

The pack was placed between the halves of the chamber with the platensbeing maintained at about 340° C. A vacuum was drawn in the enclosedspace and the pack was left for 10 minutes before applying a pressure of1 atmosphere over a period of 5 minutes to the upper diaphragm of thepack to mould the slab into the female mould. After 60 minutes thepressure was released. The foam slab was removed from between thediaphragms and found to have taken up the shape of the ash tray mouldwith precision. The surface quality was good, particularly on the sidecontacting the mould.

I claim:
 1. A method of forming a shaped article of preformed,thermoplastic, syntactic foam material of hollow microspheres in athermoplastic polymer matrix comprising:(a) contacting at least onesource of the preformed syntactic foam with a plastically deformablediaphragm member; (b) heating the assembled foam and diaphragm member toa temperature above the softening point of the thermoplastic polymermatrix; (c) applying differential pressure across the assembled foam anddiaphragm member to cause the foamed material to conform to a desiredshape; and (d) allowing the shaped foam to cool to a temperature belowsaid softening point whereby the foam retains the desired shape.
 2. Amethod according to claim 1 in which the foam stock is formed whilstconfined between a pair of plastically deformable diaphragms.
 3. Amethod according to claim 1 wherein the plastically deformable diaphragmmembers are plastically deformable metal diaphragms.
 4. A methodaccording to claim 1 wherein the plastically deformable diaphragms areof an organic thermoplastic material which has a minimum formingtemperature equal to or greater than the flow temperature of thethermoplastic matrix of the foam stock.
 5. A method according to claim 4wherein at least one of the thermoplastically deformable diaphragmsbecomes integral with the foam stock during the shaping of the articleand forms part of the final shaped article.
 6. A method according toclaim 1 in which the foam stock is provided with a coating of fiberreinforced material before the foam stock is shaped.
 7. A methodaccording to claim 6 in which the coating is laid up in loose contactwith the foam stock and becomes integral with the foam stock as a resultof the conditions used in shaping the article.
 8. A method according toeither of claims 6 or 7 in which the fiber reinforced coating materialconsists of a fibre reinforced thermoplastic layer in which the fibresare continuous, collimated fibres interwoven or overlaid to provdiequasi-isotropic reinforcement in the layer.